The present invention relates to a process and apparatus for manufacturing optical fibers (the term "optical" covering the infrared, visible and ultraviolet ranges of the spectrum) and it finds a particularly important application, although not exclusive, in the manufacture of optical fibers for telecommunication networks.
Optical fibers must fulfil two conditions, which are to a certain extent contradictory, for them to be suitable for use in a telecommunication network. Their attenuation must be low; it is considered that it must not exceed 3 to 4 dB/km at 0.85 micron (and preferably not exceed 2 dB/km at 1.3 micron). Their passband must reach about 200 MHz.km, which rules out step index fibers. The method of manufacture must be adapted to fabrication of large quantities at a moderate cost and it is expected that the requirements may be as high as one million kilometers per year.
At the present time, monomode fibers are scarcely used. Multimode fibers are either of the stepped index type, which comprises a central core surrounded by a cladding having a constant index but different from that of the core, or of the graded index type in which the cladding material has an index which varies in the radial direction.
Numerous methods are known for manufacturing optical fibers. None completely fulfils the above conditions.
The vapour-phase chemical deposition methods use heating means for growing a layer of material (generally doped silica) either radially on a tube or axially. Such methods provide step index or graded index fibers whose optical performance is largely sufficient for the contemplated applications. But the manufacturing costs are high, particularly because the whole body of the fiber (core and cladding) is manufactured by costly processes ill-adapted to mass production.
Those manufacturing methods which use glass-making techniques from material of high purity, using a double crucible (as disclosed for instance in U.S. Pat. No. 2,992,517 to Hicks), have the advantage of continuous operation. The phase-separation techniques have the disadvantage of being ill-adapted to the production of sufficiently pure materials from inexpensive base material which include impurities. The techniques for mass preparation of silica by reaction in a very-high-temperature plasma may also be mentioned. The latter process seems of particular interest since large ingots of silica having a good optical quality may be obtained at low cost from inexpensive base materials and the high temperature of the reaction in the plasma ensures purification. On the other hand, that high temperature prohibits the addition of numerous index varying dopants which are too volatile: at the present time, silica may only be doped with fluorine, which lowers its index. Finally, this process seems only applicable to the manufacture of stepped index fibers, comprising a pure or fluorine-doped silica core and a cladding formed by a plastic sheath, such as silicone. But such a stepped index fiber has a passband width limited to about 30 MHz.km, which is too low if the fiber is to be used for conveying data over distances reaching several kilometers. The stability of the fiber is doubtful since the optical interface is provided by a plastic material which is sensitive to external factors, such as moistness and temperature.
It is an object of the invention to provide a method of fabricating optical fibers adapted to manufacture of graded index fiber continuously and at moderate cost. For that purpose, a method according to the invention includes the steps of: welding bars of homogeneous silica-base material in end to end relation; progressively applying a silica cladding containing a proportion of index varying dopant which varies as the diameter of the cladding increases on said bars so as to form successive preforms; and drawing out the welded preforms to the fiber diameter.
The bars may be formed from pure or doped silica; the thickness of the deposit and the law of index profile variation will be chosen depending on the passband to be provided, and on the numerical aperture desired, by taking into account the maximum index variations which can be obtained by chemical-reaction deposition processes, particularly flame hydrolysis (also called "Outside Vapour Phase Oxidation" OVPO) which is of particular interest.
The additional steps for obtaining a directly usable fiber such as drying, application of coats, curing, storing, may be carried out continuously on the manufacturing line itself, rather than on successive batches.
The starting material for the method consists of silica bars which may be obtained at a low cost with the desired degree of purity using well-mastered techniques, typically reactive plasma. Only the cladding is manufactured by a relatively slow and costly chemical-reaction deposition process.
It is another aspect of the invention to provide an apparatus for continuous manufacture according to the above-defined process. According to another aspect of the invention, there is provided an apparatus for continuously manufacturing optical fibers, comprising: means for continuously advancing successive silica bars intended to form the core of fibers along a rectilinear path; means located on said path for welding the bars in end to end relation as their adjacent portions are moved past said means; deposition means for depositing by chemical reaction asilica base cladding containing a proportion of index varying dopant which is progressively varied as the diameter of the cladding increases and forming successive preforms; and means for drawing out the preforms to the diameter of the fiber.
The invention will be better understood from the following description of a method and an apparatus according to a particular embodiment thereof, given by way of example only. The description refers to the accompanying drawings.